1. Field of the Invention
The present invention relates generally to a lead frame for a hollow plastic package. More specifically, the present invention relates to a lead frame for a hollow plastic package which is superior in package strength, lead strength and high frequency characteristics.
2. Description of the Prior Art
Conventionally, many semiconductor elements are used with an air-tight seal in order to protect themselves from humidity, temperature, contaminant or the like which otherwise causes degradation of characteristics of the element and shortening of life, and enhance mechanical strength for improving handling ability. Kinds of package with an air-tight seal can be classified to a hollow structure package using a ceramic casing and a resin sealed package of epoxy resin or the like. As the resin sealed package, there is a package formed by transfer molding of epoxy resin or the like, and thus completely covering the semiconductor element, and a hollow package which has hollow structure not completely covering the semiconductor element in order to avoid degradation of high frequency characteristics of the semiconductor element (Japanese Unexamined Patent Publication No. Hei 4-312963).
Recently, in order to reduce cost of semiconductor devices, particularly such as compound semiconductor devices, it has been required a low cost package that does not degrade high frequency characteristics. Therefore, it has been studied to turn a resin hollow structure package to practical use, in which the compound semiconductor element is not covered completely by the resin. A lead frame to be used in the resin hollow structure package is formed by bending leads outside of the plastic casing of the package in order to permit surface mounting for any packages. However, stress is caused on the plastic casing upon bending formation of the leads to be a cause of faulty product. Thus, in order to reduce the stress, it has been attempted to make the overall leads thinner.
FIG. 1A is a section showing one example of peeling off of a conventional plastic casing, and FIG. 1B is a section showing one example of peeling off of a conventional lead. Even in an attempt to reduce the stress by making overall leads thinner, slight peeling off 401 of the casing at a portion where the plastic casing contact with the lead and peeling off 402 of the lead within the plastic casing can be caused due to stress upon bending formation of the lead of the lead frame as to a resin hollow structure package (hollow plastic package). The reason is that while the backside of the lead frame of the hollow plastic package is covered with the resin, the surface side is merely held by a ring-shaped or quadrangular ridge (wall) structure for sealing a cap later. Namely, when the overall leads are formed to be narrower, a force to depress the lead becomes smaller in comparison with the plastic package by normal transfer molding. Thus, the plastic casing and the lead cannot withstand the stress upon bending formation to cause peeling off 401 of the casing and peeling off of the lead 402.
On the other hand, when the overall leads are made thinner, input/output impedance becomes large, especially upon use of high frequency to cause difficulty in impedance matching when the application is used and to make it difficult to obtain a good grounding condition.